Tomographic imaging methods are characterized in that internal structures of an object under examination can be examined without invasive procedures thereby needing to be conducted. One possible type of tomographic image generation consists of taking a number of projections of the object to be examined from different angles. From these projections a two-dimensional sectional image or a three-dimensional volume image of the object under examination can be calculated.
An example of such a tomographic imaging method is computed tomography. There are a multiplicity of methods known for the scanning of an object under examination with a CT system. For example, circular scanning, sequential circular scanning with the object being moved forward, or spiral scanning are used. Other types of scanning which are not based on circular movements are also possible, such as, for example, scans with linear segments. With the aid of at least one x-ray source and at least one detector located opposite, absorption data from the object under examination is taken from different recording angles, and this absorption data or, respectively, projections gathered in this way are calculated by means of appropriate reconstruction methods such as to form sectional images through the object under examination.
For the reconstruction of computed tomographic images from x-ray-CT data records of a computed tomography device (CT device), i.e. from the projections acquired, use is nowadays made, as a standard method, of what is referred to as filtered back projection (FBP). After the acquisition of the data, it is usual for what is referred to as a “rebinning” step to be carried out, in which the data produced from the fan-shaped beam propagating from the source is re-arranged in such a way that it is present in a form as if the detector were to be impinged by x-ray beams running parallel to the detector. The data is then transformed into the frequency range. A filtering process takes place in the frequency range, and the filtered data is then transformed back. With the aid of the thus resorted and filtered data, a back projection onto the individual voxels then takes place within the volume of interest. However, due to their approximative mode of operation, problems arise with the traditional FBP methods with what are referred to as low-frequency cone-beam artifacts and spiral artifacts. In addition to this, with traditional FBP methods the sharpness of the image is linked to the image noise. The higher the sharpness achieved, the higher also the image noise, and vice-versa.
The FBP method belongs to the group of approximative reconstruction methods. There also exists the group of exact reconstruction methods, but at the present time these are scarcely used. Finally, a third group of reconstruction methods is formed by the iterative methods.
Due to the expansion of the detector, there is a limited measuring range available, the field of view. This means that, at a specific projection angle, projection data or measured data can be acquired only for those volume elements of an object under examination which lie within the field of view. The problem frequently arises, however, that the expansion of the object under examination is of such a nature that not all the parts of the object under examination are located inside the field of view during the entire period of measured data acquisition. This leads to incomplete measured data records with regard to these parts of the object under examination, and therefore to artifacts during the image reconstruction.